Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. References disclose a thin film transistor in which such a metal oxide having semiconductor characteristics is used for a channel formation region (Patent Documents 1 to 4, and Non-Patent Document 1).
Examples of metal oxides include not only an oxide of a single metal element but also an oxide of a plurality of metal elements (multi-component oxides). For example, InGaO3(ZnO)m (m is a natural number) which is a homologous compound is a known material as multi-component oxides (Non-Patent Documents 2 to 4).
In addition, it has been proved that an oxide semiconductor including such an In—Ga—Zn-based oxide can be used as a channel layer of a thin film transistor (Patent Document 5, and Non-Patent Documents 5 and 6).